Handover control, apparatus, base station, edge router, relay router, radio terminal apparatus, mobile communication system, and handover control method

ABSTRACT

When a radio terminal unit  30  undergoes a handover between base stations  34, 36 , a previous edge router  40  having been connected to the radio terminal unit  30 , out of edge routers  40, 42  provided in respective base stations  34, 36 , provides an instruction to instruct a relay router  48 , under which the edge router  40  and every edge router  42  in the base station  36  as a candidate for a destination of the movement are connected, to buffer data from a correspondent node, and instructs the relay router to forward the data buffered in the relay router  48  according to the instruction, to the radio terminal unit  30  after the movement, after completion of the handover, whereby the buffered data is forwarded to the radio terminal unit  30 . This implements forwarding packet data through the optimal path to the radio terminal unit after the movement in the handover control and achieves effective utilization of network resources and reduction of handover transactions.

TECHNICAL FIELD

[0001] The present invention relates to a handover control apparatus, abase station, an edge router, a relay router, a radio terminal unit, amobile communication system, and a handover control method.

BACKGROUND ART

[0002] The conventional schemes proposed in IETF (Internet EngineeringTask Force) (for example, Govind Krishnamurthi, Robert C. Chalmers,Charles E. Parkins, “Buffer management for smooth handovers in ipv6.,”INTERNET DRAFT, March 2001) involve handover control to buffer packetdata.

[0003] In this handover control, packet data is buffered in an edgerouter provided in a base station during a handover of a radio terminalunit, in order to implement handover control without loss of packetdata. After completion of the handover, the buffered packet data isforwarded to an edge router at a destination.

[0004] A configuration of a mobile communication system performing suchhandover control is presented in FIG. 6 and will be described below.

[0005] This mobile communication system shown in FIG. 6 is comprised ofa radio terminal unit 10 which is a mobile communication means such as acellular telephone or the like, a plurality of base stations 12, 14, 16for radio communication with the radio terminal unit 10, edge routers18, 20, 22 provided in these base stations 12-16, and CORs (Cross OverRouters) 24, 26, 28 connected to these edge routers 18-22. Here the CORs24-28 are arranged in a hierarchical structure with COR 28 at the top,in which the edge routers 18, 20 are under COR 24, the edge router 22under COR 26, and the edge routers 18-22 under COR 28 through CORs 24,26.

[0006] In this configuration, the data buffering point was the previousedge router 20 in the handover control during movement of the radioterminal unit 10 from a covered area of the edge router 20 to a coveredarea of the edge router 22. In this handover control, all packet datatransmitted from an unrepresented correspondent node as a sender duringthe handover of the radio terminal unit 10 was once forwarded throughCORs 28, 24 to the previous edge router 20 connected before themovement, as indicated by an arrow Y1, and buffered therein.

[0007] After completion of the handover of the radio terminal unit 10 tothe edge router 22, an instruction is then given to instruct theprevious edge router 20 to forward the buffered packet data and inaccordance with this instruction, as indicated by an arrow Y2, thepacket data is forwarded from the edge router 20 through a plurality ofCORs 24, 28, 26 to the radio terminal unit 10 connected to the new edgerouter 22 after the movement. The lossless handover was achieved by thebuffering in the previous edge router 20 in this way.

DISCLOSURE OF THE INVENTION

[0008] In the conventional handover control method, however, sinceduring the handover the packet data is buffered in the previous edgerouter 20 having been connected to the radio terminal unit 10, thepacket data is forwarded from the edge router 20 connected before themovement, through the CORs 24, 28, 26 to the radio terminal unit 10connected to the edge router 22 after the movement, as indicated by thearrow Y2, during the period after the handover of the radio terminalunit 10 and before completion of the forwarding the buffered data.Namely, the packet data is forwarded through the redundant path duringthe handover control and this poses the problem of waste of networkresources.

[0009] In this handover control, there simultaneously exist two pathsfor forwarding buffered packet data: the path of code Y2 for forwardingpacket data from the edge router 20 to the edge router 22 and the pathof code Y3 for transmitting packet data from the correspondent node tothe edge router 22, as shown in FIG. 6. Therefore, the packets via therespective paths arrive in a mixed state at the edge router 22, whichraises the problem of packet miss-ordering during the handover.

[0010] The present invention has been accomplished in view of the aboveproblems and an object of the invention is to provide a handover controlapparatus, a base station, an edge router, a relay router, a radioterminal unit, a mobile communication system, and a handover controlmethod capable of implementing forwarding packet data through an optimalpath to the radio terminal unit after movement in handover control andthereby achieving effective utilization of network resources andprevention of the packet miss-ordering during the handover.

[0011] In order to solve the above problems, a handover controlapparatus of the present invention is a handover control apparatus forcontrolling handover of a radio terminal unit between edge routersprovided in respective base stations for radio communication with radioterminal unit, the handover control apparatus comprising: control meansfor performing such control as to buffer data from a correspondent nodein a relay router under which a previous edge router having beenconnected to the radio terminal unit and every edge router as acandidate for a destination of the movement are connected, during thehandover.

[0012] In this configuration, the data from the correspondent node isbuffered in the relay router, whereby it is feasible to nullify theredundant path in forwarding packet data and thereby achieve theeffective utilization of network resources and the prevention of packetmiss-ordering during the handover. In the conventional technology, sinceduring the handover the data was buffered in the previous edge routerhaving been connected to the radio terminal unit, the data was forwardedfrom the edge router connected before the movement, through theredundant path via a plurality of routers to the radio terminal unitafter the movement, which resulted in waste of network resources. Inaddition, there simultaneously existed two paths: the path forforwarding buffered packet data from the previous edge router to the newedge router after movement and the path for forwarding packet data fromthe correspondent node to the new edge router after movement; therefore,there occurred the packet miss-ordering because of delivery of thepackets in a mixed state.

[0013] In order to solve the above problems, a base station of thepresent invention is a base station for detecting handover of a radioterminal unit, the base station comprising: retaining means forretaining location information about locations of an adjacent basestation and every relay router connected to the adjacent base station;determining means for determining a relay router under which a basestation having been connected to the radio terminal unit and every basestation as a candidate for a destination of the movement are connected,based on the location information thus retained; and instructing meansfor instructing the relay router thus determined to buffer data from acorrespondent node.

[0014] In this configuration, the appropriate relay router is instructedto buffer the data from the correspondent node, whereby it is feasibleto buffer the data to be transmitted to the radio terminal unit underthe handover, and thereby prevent data reception loss.

[0015] In order to solve the above problems, an edge router of thepresent invention is an edge router provided in a base station fordetecting handover of a radio terminal unit and connected by radiocommunication to the radio terminal unit, the edge router comprising:retaining means for retaining location information about locations of anadjacent edge router and every relay router connected to the adjacentedge router; determining means for determining a relay router underwhich a previous edge router having been connected to the radio terminalunit and every edge router as a candidate for a destination of themovement are connected, based on the location information thus retained;and instructing means for instructing the relay router thus determinedto buffer data from a correspondent node.

[0016] In this configuration, the data from the correspondent node isbuffered in the relay router, whereby it is feasible to nullify theredundant path in forwarding packet data and thereby achieve theeffective utilization of network resources and the prevention of packetmiss-ordering during the handover. In the conventional technology, sinceduring the handover the data was buffered in the previous edge routerhaving been connected to the radio terminal unit, the data was forwardedfrom the edge router connected before the movement, through theredundant path via a plurality of routers to the radio terminal unitafter the movement, which resulted in waste of network resources. Inaddition, there simultaneously existed two paths: the path forforwarding buffered packet data from the previous edge router to the newedge router after movement and the path for forwarding packet data fromthe correspondent node to the new edge router after movement; therefore,there occurred the packet miss-ordering because of delivery of thepackets in a mixed state.

[0017] In order to solve the above problems, a relay router of thepresent invention is configured to buffer data from a correspondent nodein accordance with an instruction to buffer data, given by theinstructing means of the above base station or the above edge router,change a forwarding address of a packet to an address of a destinationof the radio terminal unit in accordance with a data forwardinginstruction from the radio terminal unit after the handover, put thedata with an identifier for identification of the packet, and forwardthe packet to the current address of the radio terminal unit.

[0018] In this configuration, the data to be transmitted to the radioterminal unit is buffered during the handover of the radio terminal unitand the buffered data can be properly forwarded after the movement ofthe radio terminal unit. In this configuration, the identifier isgenerated in the relay router, and the packet data transmitted from thecorrespondent node as a sender is kept unchanged without beingrewritten, whereby the packet data can be transmitted to the radiocommunication terminal unit after the movement, without being involvedin the problem of security.

[0019] In order to solve the above problems, a radio terminal unit ofthe present invention is a radio terminal unit configured to implementradio communication with each base station and, during handover betweenedge routers provided in respective base stations, perform control ofthe handover, the radio terminal unit comprising: control means forperforming such control as to make a relay router under which a previousedge router having been connected to the radio terminal unit and everyedge router as a candidate for a destination of the movement areconnected, buffer data from a correspondent node during the handover.

[0020] In this configuration, the data from the correspondent node isbuffered in the relay router, whereby it is feasible to nullify theredundant path in forwarding packet data and thereby achieve theeffective utilization of network resources and the prevention of packetmiss-ordering during the handover. In the conventional technology, sinceduring the handover the data was buffered in the previous edge routerhaving been connected to the radio terminal unit, the data was forwardedfrom the edge router connected before the movement, through theredundant path via a plurality of routers to the radio terminal unitafter the movement, which resulted in waste of network resources. Inaddition, there simultaneously existed two paths: the path forforwarding buffered packet data from the previous edge router to the newedge router after movement and the path for forwarding packet data fromthe correspondent node to the new edge router after movement; therefore,there occurred the packet miss-ordering because of delivery of thepackets in a mixed state.

[0021] In order to solve the above problems, a mobile communicationsystem of the present invention is a mobile communication system forimplementing handover of a radio terminal unit between base stations,the mobile communication system comprising: relay routers connectedabove the base stations in a hierarchical structure and configured toperform relaying of data to the base stations and buffering of data; andedge routers provided in the respective base stations, each edge routerbeing configured to provide an instruction to instruct a relay routerunder which a base station having been connected to the radio terminalunit and every base station as a candidate for a destination of themovement are connected, to buffer data from a correspondent node duringthe handover, and instruct the relay router to forward the data bufferedin the relay router according to the instruction to the radio terminalunit after the movement.

[0022] In this configuration, the data from the correspondent node isbuffered in the relay router, whereby it is feasible to nullify theredundant path in forwarding packet data and thereby achieve theeffective utilization of network resources and the prevention of packetmiss-ordering during the handover. In the conventional technology, sinceduring the handover the data was buffered in the previous edge routerhaving been connected to the radio terminal unit, the data was forwardedfrom the edge router connected before the movement, through theredundant path via a plurality of routers to the radio terminal unitafter the movement, which resulted in waste of network resources. Inaddition, there simultaneously existed two paths: the path forforwarding buffered packet data from the previous edge router to the newedge router after movement and the path for forwarding packet data fromthe correspondent node to the edge router after movement; therefore,there occurred the packet miss-ordering because of delivery of thepackets in a mixed state.

[0023] In order to solve the above problems, a handover control methodaccording to the present invention is a handover control method ofcontrolling handover of a radio terminal unit between edge routersprovided in respective base stations for radio communication with radioterminal unit, the handover control method comprising: a control step ofperforming such control as to buffer data from a correspondent node in arelay router under which a previous edge router having been connected tothe radio terminal unit and every edge router as a candidate for adestination of the movement are connected, in accordance with aninstruction to buffer the data from the correspondent node.

[0024] In this configuration, the data from the correspondent node isbuffered in the relay router, whereby it is feasible to nullify theredundant path in forwarding packet data and thereby achieve theeffective utilization of network resources and the prevention of packetmiss-ordering during the handover. In the conventional technology, sinceduring the handover the data was buffered in the previous edge routerhaving been connected to the radio terminal unit, the data was forwardedfrom the edge router connected before the movement, through theredundant path via a plurality of routers to the radio terminal unitafter the movement, which resulted in waste of network resources. Inaddition, there simultaneously existed two paths: the path forforwarding buffered packet data from the previous edge router to the newedge router after movement and the path for forwarding packet data fromthe correspondent node to the edge router after movement; therefore,there occurred the packet miss-ordering because of delivery of thepackets in a mixed state.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a block diagram showing a configuration of a mobilecommunication system according to an embodiment.

[0026]FIG. 2 is a first block diagram for explaining a method ofdetermining a relay router in a mobile communication system according toan embodiment.

[0027]FIG. 3 is a second block diagram for explaining a method ofdetermining a relay router in a mobile communication system according toan embodiment.

[0028]FIG. 4 is a block diagram for explaining the operation offorwarding packet data buffered in a relay router in a mobilecommunication system according to an embodiment.

[0029]FIG. 5 is a block diagram for explaining the operation offorwarding packet data buffered in a relay router in a mobilecommunication system according to an embodiment.

[0030]FIG. 6 is a block diagram showing the configuration of theconventional mobile communication system.

BEST MODE FOR CARRYING OUT THE INVENTION

[0031] Embodiments of the invention

[0032] An embodiment of the present invention will be described below indetail with reference to the drawings.

[0033]FIG. 1 is a block diagram showing a configuration of a mobilecommunication system according to an embodiment of the presentinvention.

[0034] This mobile communication system shown in FIG. 1 is comprised ofa radio terminal unit 30 which is a mobile communication means such as acellular phone or the like, a plurality of base stations 32, 34, 36 forradio communication with the radio terminal unit 30, edge routers 38,40, 42 provided in the respective base stations 32-36, and CORs 44, 46,48 connected to these edge routers 38-42.

[0035] Here the CORs 44-48 are arranged in a hierarchical structure withCOR 48 at the top, in which the edge routers 38, 40 are under COR 44,the edge router 42 under COR 46, and the edge routers 38-42 under COR 48through the CORs 44, 46. The edge routers 38, 40, 42 retain informationabout adjacent edge routers to which the radio terminal unit 30 isexpected to move next. For example, the edge router 40 retainsinformation about the adjacent edge routers 38, 42 as the informationabout the adjacent edge routers to which the radio terminal unit 30 isexpected to move next.

[0036] The mobile communication system is constructed such as RegionalRegistrations, Hierarchical Mobile IP proposed in IETF.

[0037] In this configuration, the handover control in movement of theradio terminal unit 30 from a covered area of the edge router 40 to acovered area of the edge router 42 is initiated according to the radiowave condition of wireless links by the radio terminal unit 30 or by theedge router 40 to which the radio terminal unit 30 is connected atpresent. At this time, the radio terminal unit 30 or the edge router 40determines the COR 48 as a relay router out of a plurality of CORs 44-48on the basis of the information about the adjacent edge routersregistered in the edge router 40.

[0038] The way of determining the relay router will be described withreference to FIGS. 2 and 3. First, as shown in FIG. 2, let us suppose asituation in which the radio terminal unit 30 is, for example, inconnection with edge router 56 in a configuration where COR 58 isconnected to each of edge routers 50, 52 provided in their respectivebase stations, COR 60 is further connected to each of edge routers 54,56, and COR 62 is connected above those CORs 58, 60 thereto.

[0039] On this occasion, if only one edge router 54 is recognized as anedge router being a candidate for a destination of movement of the radioterminal unit 30 from the adjacent edge router information retained inthe edge router 56, the COR 60, which is connected above to both of theedge router 56 in a currently connected state and the edge router 54 asa candidate destination (or under which the edge router 56 in thecurrently connected state and the edge router 54 as a candidatedestination both are connected), is determined as a relay router.Namely, the COR 60, which can transmit packet data from thecorrespondent node to both the edge router 56 in the currently connectedstate and the edge router 54 as a candidate destination and which hasthe minimum hop count from the radio terminal unit 30, is determined asa relay router. The hop count represents the distance (not physicaldistance) from the radio terminal unit to the relay router to buffer thedata.

[0040] As shown in FIG. 3, let us suppose another situation in which theradio terminal unit 30 is, for example, in connection with edge router54 in a configuration where COR 58 is connected to each of edge routers50, 52 provided in their respective base stations, COR 60 is furtherconnected to each of edge routers 54, 56, COR 62 is connected above tothose CORs 58, 60, COR 78 is further connected to each of edge routers70, 72, COR 80 is further connected to each of edge routers 74, 76, COR82 is connected above to those CORs 78, 80, and COR 84 is connectedabove to CORs 62, 82.

[0041] Let us suppose that on this occasion two edge routers 52, 56 arerecognized as edge routers being candidates for the destination ofmovement of the radio terminal unit 30 from the adjacent edge routerinformation retained in the edge router 54. In this case, the COR 62,which is connected above to both of the COR 60 connected above to theedge router 54 in a currently connected state and the edge router 56 asa candidate destination and the COR 58 connected above to the edgerouter 52 as another candidate destination, is determined as a relayrouter.

[0042] Namely, the COR 62, which can transmit the packet data from thecorrespondent node to the edge router 54 in the currently connectedstate and to all the edge routers 56, 52 as candidate destinations andwhich has the minimum hop count from the radio terminal unit 30, isdetermined as a relay router.

[0043] The radio terminal unit 30 or the edge router 40 in the currentlyconnected state to the radio terminal unit 30 sends a bufferinginstruction message to the COR (relay router) 48 shown in FIG. 1 anddetermined as described above, as indicated by an arrow Y3. The COR 48,receiving this message, buffers packet data transmitted thereafter asindicated by an arrow Y4 from the correspondent node, as a relay router.Thereafter, the radio terminal unit 30 undergoes a handover to one edgerouter 42 out of the edge routers 38, 42 of candidate destinations.

[0044] The radio terminal unit 30 after the handover to the new edgerouter 42 acquires an address used in the edge router 42 and sends alocation registration message for location registration of the newaddress through the COR (relay router) 48 to the unrepresented homenetwork and the correspondent node as indicated by an arrow Y5.

[0045] For performing this transmission, there are two conceivablemethods in the present invention: a method of separately sending alocation registration message and a forwarding instruction message; anda method of sending one message containing the both. When the forwardinginstruction message passes the COR (relay router) 48, the COR (relayrouter) 48, receiving the forwarding instruction message, capsulates thebuffered packet data on the basis of the current address of the radioterminal unit 30 and forwards the capsulated data through the edgerouter 42 to the radio terminal unit 30, as indicated by an arrow Y6.After completion of the location registration, packet data thereaftertransmitted from the source correspondent node to the radio terminalunit 30 is transmitted to the address of the destination, therebyterminating the buffering in the COR (relay router) 48.

[0046] The operation of forwarding the packet data buffered in the COR(relay router) 48 will be described below with reference to FIG. 4. Theradio terminal unit 30 with the address A for identification of theradio terminal unit in the relay router sends the buffering instructionmessage to instruct the COR (relay router) 48 to buffer the packet dataaddressed to the address A before a handover. According to thisinstruction, the COR (relay router) 48 buffers packet data addressed tothe address A from the correspondent node as indicated by code 90.

[0047] Furthermore, when the radio terminal unit 30 is assigned anaddress B after the handover, it sends to the correspondent node amessage for location registration of the address B (locationregistration message) and a message for instruction of forwarding packetdata addressed to the address A (forwarding instruction message). Thesemessages can also be sent in the form of one message.

[0048] The COR (relay router) 48, receiving those messages, capsulatesthe packet data to the address A having been buffered there, whileassigning the data the destination address B, as indicated by code 91,and then transmits it to the address B. The radio terminal unit 30,receiving the packet data 91 addressed to B, decapsulates the packetdata as indicated by code 92. Since the destination address of thedecapsulated packet data 92 at this time is A, the radio terminal unit30 cannot receive the packet data 92 under normal circumstances.However, the radio terminal unit 30 is configured to memorize theaddress A before the movement in the present invention, so that it canalso receive the packet data 92 addressed to the address A, whereby theradio terminal unit can receive the packet data to the address A havingbeen buffered in the COR (relay router) 48 during the handover.

[0049] In the mobile communication system of the embodiment, asdescribed above, on the occasion of the handover of the radio terminalunit 30 between base stations 34, 36, the previous edge router 40 havingbeen connected to the radio terminal unit 30, out of the edge routers40, 42 provided in the respective base stations 34, 36, instructs therelay router 48, under which the edge router 40 and every edge router 42of the base station 36 as a candidate destination are connected, tobuffer the packet data from the correspondent node, and instructs therelay router 48 to forward the packet data buffered in the relay router48 on the basis of the instruction, to the radio terminal unit 30 afterthe movement after the handover, and then the buffered packet data isforwarded to the radio terminal unit 30.

[0050] This permits the packet data from the correspondent node to bebuffered in the relay router 48 during the handover, whereby it isfeasible to nullify the redundant path during forwarding the packet dataand thereby achieve the effective utilization of network resources. Inaddition, there occurs no packet miss-ordering during the handover. Inthe conventional technology, since during the handover the data wasbuffered in the previous edge router having been connected to the radioterminal unit, the data was forwarded from the edge router before themovement through the redundant path via a plurality of routers to theradio terminal unit after the movement, which resulted in waste ofnetwork resources. In addition, there simultaneously existed two paths:the path for forwarding buffered packet data from the edge router beforethe movement to the edge router after the movement and the path forforwarding packet data from the correspondent node to the edge routerafter the movement; therefore, there occurred the packet miss-orderingbecause of delivery of the packets in a mixed state.

[0051] Applied to the relay router 48 to buffer the packet data from thecorrespondent node is the relay router that has the minimum hop countfrom the radio terminal unit 30 among the relay routers under which allthe candidate base stations as potential destinations are connected, outof the plurality of relay routers 44-48 connected in the hierarchicalstructure. This results in buffering the packet data from thecorrespondent node in the relay router 48 that is the relay router underwhich the base station having been connected to the radio terminal unit30 and every base station as a candidate destination are connected, andthat has the minimum hop count from the radio terminal unit, so that thepacket data can be forwarded through the shortest path to the radioterminal unit 30 after the movement, whereby it is feasible to achievemore effective utilization of network resources.

[0052] In the present embodiment, each edge router retains the locationinformation about locations of adjacent edge routers and the relayrouters connected to the adjacent edge routers, the relay router underwhich the previous edge router having being connected to the radioterminal unit 30 under a handover and every edge router as a candidatedestination are connected is determined based on the retained locationinformation, and the relay router thus determined is instructed tobuffer the packet data from the correspondent node. This results ininstructing the appropriate relay router to buffer the packet data fromthe correspondent node, at the time of detecting a handover in the basestation, whereby the packet data to be transmitted to the radio terminalunit under the handover can be buffered well without occurrence ofpacket data reception loss.

[0053] For determining the relay router, the present embodiment isprovided with the function of determining the relay router with theminimum hop count from the radio terminal unit 30 among the relayrouters under which all the candidate base stations as potentialdestinations are connected, out of the plurality of relay routersconnected in the hierarchical structure. This enables the relay routerclosest to the radio terminal unit 30 after the movement to beinstructed to buffer the packet data from the correspondent node.

[0054] The present embodiment is also provided with the function ofinstructing the relay router buffering the packet data to forward thebuffered packet data to the radio terminal unit 30, after the movementby the handover of the radio terminal unit 30. This enables theinstruction to forward the packet data from the correspondent nodehaving been buffered in the relay router with the minimum hop count fromthe radio terminal unit, which is the relay router under which the basestation having been connected before the movement to the radio terminalunit 30 and every base station as a candidate destination are connected.

[0055] The above relay router has the function of buffering the packetdata from the correspondent node in accordance with the packet databuffering instruction from the edge router, capsulating the bufferedpacket data with the destination address of the radio terminal unit 30in accordance with the packet data forwarding instruction from the edgerouter, providing it the packet identifier, and forwarding thecapsulated data to the destination address. This permits the relayrouter to buffer the packet data to be transmitted to the radio terminalunit 30, and to properly forward the buffered packet data to the radioterminal unit 30 after the movement.

[0056] The radio terminal unit 30 memorizes the address foridentification of the radio terminal unit in the relay router during thehandover, receives the capsulated data forwarded from the relay router,and, if in this received packet data there exists the same address asthe memorized address, acquires the packet data provided with the sameaddress. This permits the radio terminal unit to recognize the addressbefore the movement in the capsulated data even if the radio terminalunit 30 is provided with a new address after the movement, and the radioterminal unit can acquire the packet data provided with the address thusrecognized. Namely, the radio terminal unit can properly acquire thepacket data having been transmitted to the radio terminal unit itselfbefore the movement, even after the movement involving change of theaddress.

[0057] In addition to the above description, the edge router or theradio terminal unit generates the buffering instruction message to therelay router when there occurs a change of the relay router, when theradio terminal unit needs to change the buffer volume of the relayrouter, or when an expiration date of the buffer becomes close. It is,however, also contemplated to employ a method of notifying all the radioterminal units of the default buffer value along with RA (RouterAdvertisement), instead of letting each radio terminal unit perform themessage exchange with the relay router, when consideration is given tolarge-scale networks. The buffering instruction message is resent atpredetermined intervals before reception of a response upon actualreception of packet data {BA (Buffer Acknowledgement)}.

[0058] After sending of the BA, all the packet data to be forwarded tothe radio terminal unit is copied and buffered in the buffer in therelay router. Thereafter, the radio terminal unit notifies the relayrouter of information to clear already-received packet data {BC (BufferClear)}, thereby deleting the data up to the packet data thus notifiedof, from the buffer.

[0059] If the buffer prepared before reception of BC becomes full in therelay router, the packet data is deleted in chronological order from theoldest. The timing of sending of BC is determined for the purpose ofupdating the expiration date before the expiration date of the assignedbuffer and for the purpose of preventing the assigned buffer area frombecoming full.

[0060] In the present invention, the above method is implemented asdescribed later by inserting a sequence number or a timestamp in theheader outside each capsulated data transmitted from the relay router tothe radio terminal unit.

[0061] The capsulation technology is used for the forwarding databetween the relay router and the radio terminal unit as describedpreviously, and, because the header used for the capsulation isgenerated in the relay router, it can be freely configured without needfor consideration to the problem of security as can occur upon rewritingof packet data transmitted from the correspondent node as a sender. Forthis reason, an option header indicating a sequence number or atimestamp of arriving packet data is introduced into the header ofcapsulated data. Filled in this option is a sequence number in whicheach packet addressed to the same radio terminal unit has arrived at therelay router. Since this value can possibly increase without a limit,the modulo of a maximum value that the prepared field can take, and itis reused at intervals of a certain period.

[0062] It is also noted that similar options are essential in thebuffering instruction message and the forwarding instruction messagewhich are messages requesting an acknowledgment. Then an identifier of amessage having served as a trigger of an acknowledgment message is alsoclearly marked in each response, in order to clearly demonstrate towhich message the response is made.

[0063] The maximum sequence number or timestamp of the packets receivedbefore the handover is marked in the aforementioned forwardinginstruction message sent after the handover. The relay router, receivingthe forwarding instruction message, deletes the packet data up to thepacket data marked in the forwarding instruction message and forwardsthe packet data thereafter to the radio terminal unit. All the foregoingmessages can be sent together with the data packets or signaling packetsused in Mobile IP.

[0064] Concerning the sequence numbers, it is necessary to manage thesequence numbers on a flow-by-flow basis. However, use of timestampsrequires no flow-by-flow management and thus is effective in large-scalenetworks and others, as compared with the case using the sequencenumbers.

[0065] As described above, the relay router has the function ofproviding the identifier of data on the occasion of forwarding thebuffered data to the radio terminal unit in accordance with theforwarding instruction from the edge router, the base station, or theradio terminal unit. This permits the relay router to generate theidentifier and thus makes it feasible to freely configure the identifierwithout consideration to the problem of security as can occur uponrewriting of packet data sent from the correspondent node as a sender.

[0066] The radio terminal unit has the function of negotiating thebuffer size of data from the correspondent node with the relay router.More specifically, the radio terminal unit transmits information abouthow much volume should desirably be allocated as the buffer size of datafrom the correspondent node, to the relay router. This permits the relayrouter to allocate only the necessary buffer size and thus makes itfeasible to perform efficient data buffering.

[0067] The radio terminal unit also has the function of notifying therelay router of information about already-received data at the radioterminal unit itself, before excess over the buffer size determined bythe above negotiation in the relay router. This permits the relay routerto know which data has been received by the radio terminal unit at theforwarding address of buffered data during the buffering of data fromthe correspondent node.

[0068] The relay router also has the function of deleting data inchronological order from the oldest on the occasion of performing thedata buffering over the date buffer size determined by the negotiation.This permits the relay router to buffer data in order from the newest ifthe buffered data exceeds the buffer size during the buffering of datafrom the correspondent node.

[0069] The relay router also has the function of deletingalready-received data notified of by the radio terminal unit. Thispermits the relay router to delete data already received by the radioterminal unit, on the occasion of buffering the data from thecorrespondent node, whereby it is feasible to implement efficient databuffering.

[0070] The radio terminal unit has the function of notifying the relayrouter of the information about already-received data before thehandover, after the handover. This permits the relay router to knowwhich data has been received before the handover by the radio terminalunit at the forwarding address.

[0071] The relay router also has the function of deleting thealready-received data before the handover notified of by the radioterminal unit and forwarding data other than it to the radio terminalunit. This permits the relay router to delete the data already receivedbefore the handover by the radio terminal unit out of the buffered dataand to forward the data other than it to the radio terminal unit, andthus makes it feasible to forward only necessary data, therebyimplementing efficient data forwarding and avoiding duplicate receptionof data.

[0072] The above embodiment was configured to determine the relay routerunder which the previous edge router having been connected to the radioterminal unit 30 under the handover and every edge router as a candidatedestination are connected, based on the location information retained inthe edge router, upon detection of the handover, and to instruct therelay router thus determined to buffer the packet data from thecorrespondent node (buffering instruction), but it is also possible tomodify the configuration so as to perform the determination of the relayrouter and the buffering instruction, prior to the detection of thehandover (i.e., regardless of the detection of the handover). In thiscase, the data transmitted to the radio terminal unit 30 beforeexecution of the handover is buffered in the relay router andtransmitted through the previous edge router to the radio terminal unit30. In this case, the relay router transmits the data with an identifierthrough the previous edge router to the radio terminal unit 30.

[0073] In the above embodiment, as described with FIG. 4, the relayrouter capsulated the buffered packet data addressed to the address A,with the destination address B being added thereto and transmitted thecapsulated data to the address B, and the radio terminal unit 30received and decapsulated the packet data 91 addressed to B. However,this may also be modified, as shown in FIG. 5, so that the relay router,receiving the packet data 95 to the address A containing the address HAset at the time of registration of the radio terminal unit 30, rewritesthe packet data into the packet data addressed to the address B, usingthe address HA as a key, and transmits it to the radio terminal unit 30after the movement.

[0074] Industrial Applicability

[0075] The present invention is applicable to mobile communicationsystems and, particularly, to the handover control apparatus, basestations, edge routers, relay routers, radio terminal units, andhandover control methods used in the mobile communication systems.

1. A handover control apparatus for controlling handover of a radioterminal unit between edge routers provided in respective base stationsfor radio communication with radio terminal unit, said handover controlapparatus comprising: control means for performing such control as tobuffer data from a correspondent node in a relay router under which aprevious edge router having been connected to the radio terminal unitand every edge router as a candidate for a destination of the movementare connected, during the handover.
 2. The handover control apparatusaccording to claim 1, wherein the relay router in which the data fromthe correspondent node is buffered is a relay router with a minimum hopcount from the radio terminal unit, out of a plurality of relay routersconnected in a hierarchical structure.
 3. The handover control apparatusaccording to claim 1 or 2, wherein the control means puts the databuffered in the relay router, a destination address of the radioterminal unit and an identifier for identification of packet data andforwards the buffered data to said address.
 4. A base station fordetecting handover of a radio terminal unit, said base stationcomprising: retaining means for retaining location information aboutlocations of an adjacent base station and every relay router connectedto the adjacent base station; determining means for determining a relayrouter under which a base station having been connected to the radioterminal unit and every base station as a candidate for a destination ofthe movement are connected, based on the location information thusretained; and instructing means for instructing the relay router thusdetermined to buffer data from a correspondent node.
 5. The base stationaccording to claim 4, wherein, for determining said relay router, saiddetermining means determines a relay router with a minimum hop countfrom the radio terminal unit among relay routers under which allcandidate base stations as potential destinations are connected, out ofa plurality of relay routers connected in a hierarchical structure. 6.The base station according to claim 4 or 5, wherein, after movement byhandover of the radio terminal unit, said instructing means instructsthe relay router determined by the determining means to forward thebuffered data to the radio terminal unit after the movement.
 7. An edgerouter provided in a base station for detecting handover of a radioterminal unit and connected by radio communication to the radio terminalunit, said edge router comprising: retaining means for retaininglocation information about locations of an adjacent edge router andevery relay router connected to the adjacent edge router; determiningmeans for determining a relay router under which a previous edge routerhaving been connected to the radio terminal unit and every edge routeras a candidate for a destination of the movement are connected, based onthe location information thus retained; and instructing means forinstructing the relay router thus determined to buffer data from acorrespondent node.
 8. The edge router according to claim 7, wherein,for determining the relay router, the determining means determines arelay router with a minimum hop count from the radio terminal unit, outof a plurality of relay routers connected in a hierarchical structure.9. The edge router according to claim 7 or 8, wherein, after movement byhandover of the radio terminal unit, said instructing means instructsthe relay router determined by the determining means to forward thebuffered data to the radio terminal unit after the movement.
 10. A relayrouter configured to buffer data from a correspondent node in accordancewith an instruction to buffer data, given by the instructing means ofthe base station as set forth in one of claims 4 to 6 or the edge routeras set forth in one of claims 7 to 9; in accordance with a dataforwarding instruction given by the instructing means, buffer the datafrom the correspondent node, put the data a current address of the radioterminal unit and an identifier for identification of packet data, andforward the packet data to the current address of the radio terminalunit; after the movement of the radio terminal unit, forward the packetdata to the current address of the radio terminal unit in accordancewith the forwarding instruction.
 11. A radio terminal unit comprising:memory means for memorizing an address for identifying the radioterminal unit in a relay router during handover; and instructing meansfor, in order to change a forwarding address of packet data after thehandover in the relay router from a previous address of the radioterminal unit to a current address of the radio terminal unit,instructing the relay router to forward packet data to the currentaddress of the radio terminal unit.
 12. A radio terminal unit configuredto implement radio communication with each base station and, duringhandover between edge routers provided in respective base stations,perform control of the handover, the radio terminal unit comprising:control means for performing such control as to make a relay routerunder which a previous edge router having been connected to the radioterminal unit and every edge router as a candidate for a destination ofthe movement are connected, buffer data from a correspondent node duringthe handover.
 13. The radio terminal unit according to claim 12, whichis configured to select and apply a relay router with a minimum hopcount from the radio terminal unit, out of a plurality of relay routersconnected in a hierarchical structure, as the relay router for bufferingthe data from the correspondent node.
 14. The radio terminal unitaccording to claim 12 or 13, wherein the control means makes the relayrouter put the data buffered in the relay router, a destination addressof the radio terminal unit and an identifier for identification ofpacket data, and forward the buffered data to said address.
 15. A mobilecommunication system for implementing handover of a radio terminal unitbetween base stations, said mobile communication system comprising:relay routers connected above the base stations in a hierarchicalstructure and configured to perform relaying of data to the basestations and buffering of data; and edge routers provided in therespective base stations, each edge router being configured to providean instruction to instruct a relay router under which a base stationhaving been connected to the radio terminal unit and every base stationas a candidate for a destination of the movement are connected, tobuffer data from a correspondent node during the handover, and instructthe relay router to forward the data buffered in the relay router inaccordance with the instruction to the radio terminal unit after themovement.
 16. The mobile communication system according to claim 15,wherein the relay router buffers packet data therein in accordance withthe instruction to buffer packet data from the edge router, forwards thepacket data with an identifier of data to the radio terminal unit, andforwards the packet data to a current address of the radio terminal unitin accordance with a packet data forwarding instruction of the radioterminal unit after the handover.
 17. The mobile communication systemaccording to claim 15 or 16, wherein the radio terminal unit negotiatesa buffer size of data from the correspondent node with the relay router.18. The mobile communication system according to claim 17, wherein theradio terminal unit notifies the relay router of information aboutalready-received data in the radio terminal unit itself, before excessover the buffer size determined by the negotiation with the relayrouter.
 19. The mobile communication system according to claim 17,wherein the relay router deletes the data in chronological order fromthe oldest during an operation of buffering data over the buffer sizedetermined by the negotiation.
 20. The mobile communication systemaccording to claim 18, wherein the relay router deletes thealready-received data notified of by the radio terminal unit and updatesan expiration date of a buffer area allocated for the radio terminalunit.
 21. The mobile communication system according to claim 18, whereinthe relay router deletes a buffer area whose expiration date has passed.22. The mobile communication system according to one of claims 16 to 21,wherein after the handover, the radio terminal unit notifies the relayrouter of information about data having already been received before thehandover.
 23. The mobile communication system according to claim 22,wherein the relay router deletes the already-received data havingalready been received before the handover, notified of by the radioterminal unit, and forwards data except said already-received data tothe radio terminal unit.
 24. A handover control method of controllinghandover of a radio terminal unit between edge routers provided inrespective base stations for radio communication with radio terminalunit, said handover control method comprising: a control step ofperforming such control as to buffer data from a correspondent node in arelay router under which a previous edge router having been connected tothe radio terminal unit and every edge router as a candidate for adestination of the movement are connected, in accordance with aninstruction to buffer the data from the correspondent node.
 25. Thehandover control method according to claim 24, wherein a relay routerwith a minimum hop count from the radio terminal unit out of a pluralityof relay routers connected in a hierarchical structure is applied to therelay router for buffering the data from the correspondent node.
 26. Thehandover control method according to claim 24 or 25, wherein saidcontrol step is to assign the data buffered in the relay router, acurrent address of the radio terminal unit and forward the buffered datato said address.
 27. The handover control method according to claim 26,wherein said control step is to assign the data an identifier of data onthe occasion of forwarding the buffered data.
 28. The handover controlmethod according to claim 26 or 27, wherein said control step is toconduct a negotiation about a buffer size of the data from thecorrespondent node in the relay router.
 29. The handover control methodaccording to claim 28, wherein said control step is to delete the datain chronological order from the oldest during an operation of bufferingthe data over the buffer size determined by the negotiation.
 30. Thehandover control method according to claim 28, wherein said control stepis to delete already-received data notified of by the radio terminalunit.
 31. The handover control method according to one of claims 26 to30, wherein said control step is to, after the handover of the radioterminal unit, delete already-received data having already been receivedbefore the handover by the radio terminal unit, from the data bufferedin the relay router and forward data except for the already-receiveddata to the radio terminal unit.